This invention relates to vacuum processing of semiconductor wafers and, more particularly, to novel methods and apparatus for removing contaminants from an ion implantation system.
Ion implantation has become a standard technique for introducing impurities into semiconductor wafers. A beam of ions is generated in a source and directed with varying degrees of acceleration toward a target wafer. Impurities are deposited into the bulk of the semiconductor wafers by using the momentum of energetic ions as a means of embedding them in the crystalline lattice of the semiconductor material. Typically, the ion beam has dimensions smaller than the semiconductor wafer, thereby requiring movement of the ion beam relative to the semiconductor wafer to obtain uniform coverage of the wafer surface. The relative movement can be effected by mechanical movement of the wafer, by deflection of the ion beam or both. Furthermore, the wafers can be presented to the ion beam either one at a time (serial processing) or in a batch mode. In one batch system, up to 25 wafers are mounted on a disk which is rotated relative to an ion beam.
At least some overscan is required in serial process systems to insure dose uniformity. In batch systems, the ion beam is scanned over an annular portion of the rotating disk which includes the target wafers. Thus, impurity ions are deposited or implanted not only on the wafer but also on the wafer holding mechanism and on the walls of the ion implantation chamber adjacent to the wafers. With time, these impurity deposits can accumulate substantially. In subsequent ion implantations, the deposits are sputtered by the ion beam and evaporated due to the elevated temperatures caused by the ion beam. A portion of the sputtered and evaporated deposits find their way to the surface of the semiconductor wafer and appear as contaminants thereon.
In the fabrication of LSI and VLSI circuits, the individual devices making up the circuits are exceedingly small and require carefully controlled processing. The dosage of implanted impurities must be tightly controlled to insure proper operating parameters. Furthermore, even a small amount of contamination on such small devices can cause degradation or destruction of the circuit. Thus, the above-described problem of deposits on the walls and other components of an ion implantation chamber has been substantial. The problem is compounded by the fact that various species of ions are run in an implantation system at different times. Thus, the deposits can contain several different species and, in general, are not the same as the species being implanted.
A number of techniques have been developed to alleviate the problem of contamination in an ion implantation system. Some users limit an ion implantation system to running only one ion species. However, this technique requires several expensive ion implantation systems to run several species and limits production flexibility. Removable vacuum chamber liners have also been utilized. However, the liners are expensive, and replacement is a time-consuming process. Still other techniques have involved physical cleaning of the chamber to remove deposits. Mechanical scrubbing and solvent cleaning have been used. Either method is time-consuming and presents safety problems, since the deposits are likely to be toxic. Ion beam sputtering has been used to remove the deposits. This technique has not been effective, since the deposits are spread around the vacuum chamber rather than removed.
It is a general object of the present invention to provide novel methods and apparatus for reducing contamination in a vacuum processing chamber.
It is another object of the present invention to provide methods and apparatus for removing contaminants from a vacuum processing chamber which do not require parts removal or disassembly of the processing chamber.
It is yet another object of the present invention to provide methods and apparatus for removing contaminants from a vacuum processing chamber in a short time.
It is still another object of the present invention to provide methods and apparatus for removing contaminants from a vacuum processing chamber in which the safety of operating personnel is not endangered.